An airbag device fitted to a driver seat and a passenger seat of a vehicle is to protect an occupant by rapidly expanding an airbag by means of a gas pressure let out from an inflator at the time of collision of the vehicle. In the airbag device described above, it is necessary to ease influence of impact given to the occupant when the airbag is expanded, thereby receiving the occupant softly by the airbag. To that end, it is preferable to avoid a situation where the airbag is abruptly expanded to protrude toward the occupant when the inflator operates. Particularly, since the occupant comes into contact with the airbag in a short time in the case where the occupant sits down adjacent to the airbag device, it is preferable to avoid a situation where the airbag is abruptly expanded to protrude toward the occupant.
For a device for controlling the expansion speed of the airbag described above, various technologies for adjusting opening and closing degree of a vent hole of the airbag are developed. For example, JP 11-334521A (patent reference 1) proposes a device for opening and closing the vent hole by an actuator. In this example, there are provided an actuator made of a plate-like piezoelectric element and a vent hole openable and closable by the actuator, respectively, formed on a retainer for supporting an inflator and an airbag, wherein an internal pressure at the beginning of the expansion of the airbag is detected by a pressure sensor, and it is decided that the occupant approaches the airbag when the internal pressure is higher than a pressure at normal time, thereby energizing the actuator to operate it, so that the vent hole is opened through which gas inside the airbag is discharged outward.
Further, JP 11-105664A (patent reference 2) proposes a device for closing a vent hole by a valve for regulating volume of gas to be let out which is formed of the same base cloth as an airbag. In this example, when the valve for regulating volume of gas to be discharged is temporarily stopped by a tacking thread and the like, a state where the vent hole keeps open is maintained, thereby controlling an expansion speed at the beginning of the expansion of the airbag. Thereafter, when the airbag is expanded, and a stretching force of the airbag becomes larger than a tacking force of the tacking thread, the tacking thread is cut so that the vent hole is throttled by the valve for regulating volume of gas to be discharged. As a result, the gas inside the airbag is restrained from being discharged outward so that the airbag is expanded until it becomes a suitable size to protect the occupant.
However, since the device in the patent reference 1 has a structure to close the vent hole by operating the actuator made of the plate-like piezoelectric element and the like, there is a drawback in that a mechanism to close the vent hole is complex and expensive, and also a response speed is slow. Further, since the patent reference 2 has a system to close the vent hole by cutting the tacking thread by the stretching force of the airbag, there is a drawback in that timing for closing the vent hole is varied largely due to the variations of a breaking force of the tacking thread and the stretching force of the airbag, and also the timing for closing the vent hole can not be changed in response to a driving posture of the occupant.